Pictures like these never quite fade. The iconic images of Hurricane Katrina—the sodden Superdome with its peeling roof, the corpses floating in the street, the hapless crowds waiting for help that wouldn't come—were enough to nudge the September 11 snapshots from the center of our brains. They were also enough to move the national debate back in the direction of domestic politics: to poverty, race, and whether a functioning government is worth paying for.

But it was another set of pictures—less seen but just as eerie—that foreshadowed the rest of the century. These were satellite images released in late September by the scientists charged with keeping track of ice in the Arctic. What they showed was simple: more water, less ice—20 percent less than the historical average. As climate change pushes polar temperatures higher, each summer's melt takes a bigger toll—and by last year, the scientists were saying, it seemed to have become a self-reinforcing process. That is, there was less white ice to reflect sunlight out to space and more blue water to absorb the heat, water that in turn melted more ice. "This will be four Septembers in a row that we've seen a downward trend," Mark Serreze, a scientist at the National Snow and Ice Data Center, told Britain's Independent newspaper. "The feeling is we are reaching a tipping point or threshold beyond which sea ice will not recover."

By itself, of course, the extent of Arctic ice affects relatively few of us. But it's not a singular case; it's a highly visible manifestation of the chaos we've let loose on the planet's physical systems. The waterlogged horror in the Gulf of Mexico was another. Katrina was not an isolated natural disaster but an obvious sign of a future far less predictable than the past.

NO SINGLE HURRICANE is the result of global warming. But a month before Katrina hit, Massachusetts Institute of Technology hurricane specialist Kerry Emmanuel published a landmark paper in the British science magazine Nature showing that in the past half century tropical storms have been lasting 60 percent longer and spinning winds 50 percent more powerful. Two weeks after Katrina struck, Science published another paper, by a Georgia Institute of Technology team, demonstrating that Category 4 and 5 hurricanes have become twice as likely in recent decades. The cause: the ever-warmer tropical seas on which they thrive. Katrina, a Category 1 storm when it crossed Florida, roared to monster size in the abnormally hot waters of the Gulf. Within a day, a million people were homeless. That's far too many, we soon found out, for the federal government to cope with. And it's just the beginning.

More than a decade ago, environmental researcher Norman Myers began adding up the number of humans at risk of losing their homes to global warming. In coastal China, India, Bangladesh, the tiny island states of the Pacific and Indian Oceans, the Nile delta, Mozambique, and other areas, he predicted that 150 million people could become "environmental refugees" by 2050, displaced by rising waters. That's more than the total number of political refugees sent scurrying by the bloody century we've just endured. Or, put another way, it's 150 Katrinas, most of them in places without Superdomes, buses, or the odd $200 billion to make repairs.

The bulk of those refugees will be in countries where nobody emits much carbon. (Global warming is manmade, but that doesn't mean it's made by all men.) The United Nations can hardly calculate Bangladesh's emissions—the country uses so little fossil fuel that they're a rounding error. Even China, with its booming economy, consumes only about an eighth as much energy per capita as the United States does. Its car-to-person ratio, though growing fast, is the same as ours was in 1912. Meanwhile, the 4 percent of the world's population living in the United States produces 25 percent of the planet's carbon dioxide.

Now the bill is coming due, with cruel poetic justice for some. Katrina cast up ironies like so much flotsam and jetsam. The storm, for instance, wreaked particular vengeance on Mississippi, a state governed by Haley Barbour (R), who in an earlier incarnation as a GOP power broker and energy lobbyist helped persuade President George W. Bush to renege on his promise to treat carbon dioxide as a pollutant. Thanks to Barbour and the other lobbyists-turned-administrators who throng the White House, the federal government has done exactly nothing to slow the progress of climate change. Today we're emitting 20 percent more carbon than we were in 1990, when scientists were first issuing their prescient warnings.

More ironies: The sky-high gasoline prices that followed Katrina actually reduced the amount of energy Americans used, at least for a few weeks, something that hadn't happened since the Carter administration. And the scores of Explorers and Navigators running out of gas on Houston freeways as their owners fled Hurricane Rita may have marked the beginning of the end of the SUV era.

Of course, it's too late to prevent global warming. So far we've raised the temperature of the planet by about one degree, globally averaged, and that's been enough to set the poles to melting and the winds to roaring. Even if, right now, we started doing all that we could to overhaul our energy economy, we'd probably still be stuck with a couple more degrees of warming, and the world would change more profoundly than at any time in human history.

But hope of progress in Washington is nil. The criminally lazy energy bill signed by Bush a few weeks before Katrina struck enshrined our current approach to energy, which, at its essence, can be summed up as "more": more drilling, more refining, more combustion, and more handouts to the oil and gas industry. Growing numbers of state and local governments have begun to act, and their efforts hold promise for the future. In the meantime, what do we have to look forward to? The computer modelers tell us that the mid-case estimate—not the worst-case but the most likely scenario for the rest of the century—is in the range of another five degrees of warming. That would make the planet hotter than it has been through all of human history.

ALL OF THIS WILL TAKE TIME to sink in. We're not used to the idea that the earth is shifting beneath us. For 10,000 years of human civilization, we've relied on the planet's basic physical stability. Sure, there have been hurricanes and droughts and volcanoes and tsunamis, but averaged out, it's been a remarkably stable run. Unfortunately, stability is a thing of the past.

We will soon learn, for example, that what we've been calling "global warming" is better thought of as excess energy trapped in the atmosphere, which will express itself in every possible way. Like the Bush administration's energy bill, these manifestations will also be about "more": more evaporation in arid lands and then more flooding when it eventually rains; more wind as air pressure rises from warmer areas; more extreme heat waves like the one that killed tens of thousands of Europeans in 2003 or the one that cut North American grain yields by a third in 1988; more ecological disruption as summers lengthen, winters shorten, and sea levels rise; more disease as mosquitoes spread to once-cool climes; and even more nonlinear surprises like the possible shutdown of the Gulf Stream.

Katrina revealed deep helplessness among our rulers. Part of it stemmed from cronyism and incompetence, part from the sheer overwhelming force of the blow. We will slowly recover, but even the United States has only so many hundreds of billions to spare. New Orleans will be rebuilt—this time. But what if hurricanes like Katrina go from being once-in-a-century storms to once-in-a-decade-or-two storms? How many times will we rebuild?

A final picture that haunts me is one I first saw as a young man: the photograph taken by the early Apollo astronauts from outer space that showed our blue and white orb against the endless black background of the universe. The picture that, for a moment, illuminated the essential borderlessness of our planet, and helped launch the environmental movement.

What haunts me is that the picture is no longer accurate. It's like a college-yearbook photo of a man now middle-aged. In the few decades since it was taken, we've changed the earth in profound ways. Much of that northern ice has turned to water. The deserts have grown. And now parts of the Gulf Coast have sunk beneath the sea. It's a new, less friendly world, and it needs its own numbering system. So let Katrina mark Year One of our new calendar, the start of an age in which the physical world has flipped from sure and secure to volatile and unhinged.

Bill McKibben is a scholar-in-residence at Middlebury College and the author of The End of Nature and Wandering Home.

THE SOLUTION TO GLOBAL WARMING is not a mystery. To cool the planet, we have to stop pumping carbon dioxide and other greenhouse gases into the atmosphere. The Sierra Club is engaged in a major effort to usher in a clean-energy future, trying to raise fuel-economy standards for new cars and campaigning for an energy policy based on wind, solar, and efficiency. To learn what you can do, visit sierraclub.org/globalwarming.